For all intensive purposes, your file is corrupted. You need to do some digging to figure out what encoding it was using if you have any hope of fixing it. In some cases there isn't much to be done except use manual find/replace in text editor. Especially for something which is already epub, as there is a good chance there the file is now using multiple encodings - UTF-8 plus whatever the original encoding was before the epub was created.

A correction I wasn't actually aware of, unlike other times where my fingers seem to do the spelling instead of my brain. That said, I think I'll join the side of the debate that says that the english language has moved on, and 'intents' is the archaic/formal form of the phrase

Anyway I really did sort of mean 'intensive' vs. 'intents', in the sense that the OP is going to probably have to bang their head against a wall or its' virtual equivalent to fix their file.